


Lost Control

by BlackWolfSpirit



Category: Undertale (Video Game)
Genre: Angst and Hurt/Comfort, Animal Instincts, Blood and Injury, Breeding, But Not Much, Cervical Penetration, Dirty Talk, Doggy Style, Emotional Hurt/Comfort, F/M, First time I've ever written a fan fic, Forgive Me, How Do I Tag, Hurt/Comfort, I have no idea what I'm doing, Knotting, Mating Cycles/In Heat, Never written smutt before, Reader has a vagina, Sans tries, Sans/anon, Slight Inflation, Smutt, Vaginal Sex, my own head canons about the skeleton monster race, porn with a little plot, sans/reader - Freeform, skeleton heat cycles
Language: English
Status: Completed
Published: 2020-09-08
Updated: 2020-09-08
Packaged: 2021-03-06 19:26:43
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 7,105
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/26354161
Author URL: https://archiveofourown.org/users/BlackWolfSpirit/pseuds/BlackWolfSpirit
Summary: Each monster species has its own unique way in which mating cycles are carried out.With skeleton monsters, the method can be a little different....There's a reason the skeleton race has always been low in numbers.
Relationships: Sans (Undertale)/Reader
Comments: 28
Kudos: 365





	Lost Control

**Author's Note:**

> I've never written fan fiction before, let alone smut. This is honestly kind of an experiment to see if I can get back into writing. I'd happily accept any critiques. I literally have no idea what I'm doing.
> 
> Song that partially inspired this. https://youtu.be/vi6v0MOWp2Q
> 
> Please go give my beautiful bata reader Bex some love. You can find their blog on Instagram @Bluexcanary!

Sunlight streamed through the open window in the guest bedroom. Trying to turn away from the invasive light hitting your eyes, you hummed to yourself as the sweet return of sleep began to wrap around your mind once more.

That calm was interrupted by the blaring of your phone's alarm, signaling the unfortunate return to the daily routine. Work, schoolwork, Sleep, repeat. Or so it usually would be, but these past few months, honestly, things had really taken a turn for you.

Sure, you still were working your nine-to-five job and had a ton of online studies to get done before ever-repeating deadlines came to interrupt what little sleep you could get.

But really– your life had been flipped upside down the day you met the skeleton brothers. One had turned into your dearest and closest friend.

The other… somehow, over the past few months had become more, and before you knew it you two were happily going steady.

Papyrus liked to refer to you too as “Datemates” no matter how much you insisted it embarrassed the hell out of you.

Swinging the covers to the side, you stretched and heard a few joints pop along the way. Sighing, you made your way down to the kitchen to grab some coffee before beginning your morning hygiene process. 

The brothers had been kind enough to set up a (more or less) permanent guest space for you. Over the past few months of knowing them, you had been spending so much time there that you practically lived at their home fifty percent of the time now.

Sans simply stated they had been meaning to officially set it up for visiting friends anyway, and that this was a good excuse. Surprisingly enough, he actually helped out and put considerable effort into gathering, assembling, and placing the room appliances. A bed, desk, chest of doors, even a little personal TV. 

That’s one thing you loved about the adorable goof. Sure, he was a lazy-ass most of the time but he seemed to put a ton of effort into the things that really mattered to him, not that he would ever admit it. 

That was another reason his lack of self-care frustrated you so. But you honestly couldn't claim much better. You were both peas in a pod in that regard. It was yet one more thing you oddly seemed to bond over– the mutual need for comfort on both ends. 

…

The clatter of plates jolted you out of your thoughts as you decended the stairs, and your nose was greeted by the scent of pancakes.

Ah, Paps was up already and practicing his breakfast culinary skills. God, it smelled good.

The lanky skeleton maintained the belief that his dinner cooking abilities were pure perfection, but admitted that in breakfast foods, he needed some training and had agreed to weekly cooking classes with Toriel. From the smell of things, they were definitely paying off!

Arriving at the coffee pot, you began brewing a cup of your favorite warm morning beverage. 

“Hey Paps, Good Morning. Smells amazing!”

“WHY THANK YOU Y/N! I THINK THEY ARE COMING ALONG QUITE NICELY. ONLY HALF OF THEM SO FAR HAVE BEEN BLACK!

“Hmmm” you chuckled as you stared at the coffee maker, silently wishing you could make it heat up faster. 

At long last you filled your cup, adding your usual coffee additions, and made your way to the dining room table to scroll through your phone briefly before getting ready for work.

In your sleepy haze, it suddenly dawned on you that your boyfriend was missing.

It didn't surprise you much, neither of you were morning people. If it wasn’t for work, you’d be in bed until eleven and probably sneak into his room to cuddle if you were being completely honest with yourself. The poor guy had crashed as soon as he got home from his job yesterday and you didn’t have the heart to disturb him if he was actually getting a decent night’s rest for a change. 

“Hey Pap, should I wake Sans up for breakfast?”

"OH… ACTUALLY, BEST FRIEND…

DO YOU HAVE A MINUTE TO CHAT, JUST THE TWO OF US?"

"Oh, sure Paps what’s up?"

"MMM… I APOLOGISE FOR THE BLUNTLESS OF THIS INQUARIY, BUT HAS MY BROTHER HAD A TALK WITH YOU ABOUT SKELETON MATING CYCLES?"

You spat out your coffee and choked on the rest still remaining in your mouth. 

"COUGH Uh…. COUGH. N-no. No, he hasn’t. I uh… I know monsters go through some sort of specific mating… thing? But every time I’ve tried to question him on the subject, he tells me not to worry and to forget about it.” 

Papyrus proceeded to comedically facepalm.

“NYEHHHH…. OF COURSE, HE DID…”

“IM SORRY HUMAN, BUT IF MY BROTHER HASN’T BROUGHT IT UP ON HIS OWN AT THIS POINT, I FEEL IT IS MY DUTY AS BOTH A GOOD BROTHER AND YOUR FRIEND TO WACH OUT FOR YOUR COMBINED SAFETY."

"Safety?"

“YES...” He began to twiddle his thumbs while trying to find a particularly interesting piece of tile on the floor to stare at. “…YOU SEE, ALL MONSTERS GO THROUGH MATING CYLES AS I SURMISE YOU ARE ALREADY AWARE, BUT EACH SPECIES HAS A SLIGHTLY DIFFERENT WAY IN WHICH IT IS CARRIED OUT.” He coughed awkwardly. 

“THERE IS A REASON THE SKELETON MONSTER RACE WAS SO FEW IN NUMBER, EVEN BEFORE THE GREAT WAR. THIS IS BECAUSE SKELETONS CAN ONLY PRODUCE ONE OFFSPRING IN THEIR LIFETIME. THIS APPLIES TO BOTH MALES AND FEMALES.”

“ONCE TWO INDIVIDUALS COME TOGETHER AND… MAKE LOVE, THERE CAN BE PHYSICAL CHANGES THAT OCCUR FOR BOTH MEMBERS.” 

"AFTER A FEMALE HAS MATED, SHE IS THEN ABLE TO GO INTO… HMM. WELL, I SUPPOSE YOU COULD CALL IT A HEAT OR AN ES-"

“Estrus cycle?”

"YES! VERY GOOD, I BELIEVE IT COULD BE CONSIDERED SIMILAR TO THAT."  


"AS I WAS SAYING, FEMALES WILL THEN GO INTO A SORT OF HEAT. THIS CAN HAPPEN ANYWHERE FROM A DAY AFTER THE PAIR’S FIRST MATTING OR A FEW MONTHS PAST THE JOINING. IT DEPENDS ON THE INDIVIDUAL, OR SO I’VE HEARD."

"ONCE FEMALES GO INTO THESE HEATS, THE MALES WILL GO INTO A SORT OF RUT THAT IS TRIGGERED BY BEING IN CLOSE PROXIMITY TO THEIR RESPECTED MATE."  


"DURING WHICH THE MALES CAN BECOME… WELL FOR LACK OF A BETTER WORD, CARNAL.  


ANAMALISTIC, AND VERY TERRITORIAL. THIS IS WHY IT IS IMPERATIVE THAT I FEEL I MUST BRING THIS TO YOUR ATTENTION AND IT WAS PURE STUPIDITY THAT MY BROTHER HASN’T! ESPECIALLY SINCE I KNOW YOU TWO HAVE STARTED TO MATE."

You almost choked again.

"W-what!?"

"OH PLEASE. THESE WALLS AREN'T SOUNDPROOF," Papyrus said as he motioned his hand around the room.

"Papyrus I am so, so sorry! I thought we were more quiet!"

"OH, NONSENSE! I DON’T REALLY MIND; I AM VERY HAPPY FOR YOU AND SANS. PLUS, I HAVE HEADPHONES TO KEEP THE SOUND OUT AND I COULD ALWAYS GO TO UNDYNE'S IF IT REALLY BOTHERED ME."  


He shook his head, focusing back on the topic at hand.

"ANYWAY, THE REASON MALES BECOME SO CARNAL IS TO; ONE, ENSURE THAT THEIR MATE IS EMBUED WITH THEIR SCENT AND MAGIC, AND TWO– THE MAIN REASON THAT THIS CAN BE DANGEROUS– IF ANOTHER MALE IS NEAR THEIR MATE, THEY WILL BE FILLED WITH THE NEED TO FEND OFF THE POTENTIAL THREAT TO THE CLAIM TO PASS DOWN THEIR, WELL… GENES SORT OF." 

"A LOT LIKE HOW MALES OF THE ANIMAL KINGDOM FIGHT FOR MATES I SUPPOSE."

"Okay…. I guess that makes sense," you replied.

"OF COURSE, A MATED PAIR CAN ONLY PRODUCE OFFSPRING AFTER A SOUL BOND IS INITIATED. THEN THE HEAT AND RUT CYCLES WILL CEASE AS THERE WILL BE NO FURTHER NEED FOR THEM."

"Wait… don't monster couples date and break up though, just like humans do? How do they deal with these cycles?" 

Papyrus seemed delighted that he could be of so much help today, despite the uncomfortable subject.

"OH! GOOD QUESTION, BEST FRIEND! MOST MONSTER FEMALES THAT NOTICE THEY'RE BEGINING A HEAT CYCLE WILL OFTEN TAKE TIME OFF WORK AND STAY INDOORS FOR A FEW DAYS TO KEEP DISTANCE FROM THEY'RE MATE. PHONE AND VIDEO CHATS CAN BE PERFECTLY SAFE TO KEEP IN TOUCH I BELIEVE, AS THE MALE WILL NOT BE ABLE TO PICK UP HER SCENT."

"But if I’m human…. how does this all come into play?"

"WELL… I'M NOT CERTAIN SINCE HUMANS FEMALES DON’T GO INTO HEATS, BUT HIS RUT COULD BE TRIGGERED BY YOUR OVULATION CYCLE." 

"I APOLOGISE AGAIN, I'M SURE THIS IS NOT THE MOST COMFORTABLE TOPIC."

You gulped, trying to swallow down your embarrassment.  


"No, no... I’m glad you told me. God… I can’t believe he never shared any of this with me before."

"AS AM I! THE LAZY BONES!"

Just then as if being summoned, the man himself emerged from the kitchen threshold grasping a steaming mug of coffee he probably poured from the remainders of your brew. 

“Hey guys” He yawned, absent-mindedly scratching at his sternum. 

“what ya chatting about?”

You and Papyrus turned your heads in unison to glare at the shorter monster 

A few beads of sweat began to appear on his cranium.

“oh, no...”

–––––––––––– 

It had been a at least a month since that awkward, yet informative morning meeting with Papyrus. He had made his poor older brother sit through another lengthy mating cycle explanation and a thorough scolding about how he had to inform you because someone was “too lazy and irresponsible to do it himself.” 

Your poor boney boyfriend was so blue in the face at that point that you were afraid he’d pass out from the amount of magic he was exerting through pure perspiration. 

In the end, the matter was dropped and not picked up again by any of you.

Admittedly, the two of you had slept together at least once since that conversation with no lasting grievances. You had come home from work, exhausted and frustrated. Sans, bless his soul, had taken the time to run a bath for you and pamper you into forgetting about the rough day altogether. 

The bath may have gotten a little out of hand at the end, but you really needed the outlet at that point and he was more than willing to help out.

\---

Huffing to yourself, you grabbed your bag and now empty coffee cup and headed to the brothers' front door. Holding the rim of the lid in your teeth while you balanced the bag in one hand, you used the other to rummage through your purse, finally acquiring the keys to unlock the door and head in. 

Papyrus had planned a hiking trip with Alphys and Undyne, and would be gone for the remainder of the week. Though they had invited you and Sans, the two of you were unfortunately tied to the unforgiving strings of work, and with so little prior notice neither of you could get the time off. 

This though did provide you and him a little quality time in the house together, and you gladly took the suggestion Pap offered of keeping his brother company while he was away and to make sure he actually took care of himself in Papyrus's absence. 

You groaned to yourself while shuffling into the living room. Half-heartedly shedding your shoes and dumping your belongings on the counter. 

In a sleepy haze you wandered into the kitchen, maybe more coffee would help wake you up enough to start on that essay you had to get done by tomorrow evening (which you had been procrastinating on).

The joys of not being bombarded by period cramps was relieving to say the least since your cycle had ended two days ago and you were reaping in the rewards of drinking way too much caffeine than was probably advisable.

Just as you were about to begin brewing a mug, you heard a soft rustle from the living room. That was odd, the house should be empty, Sans didn't get off until six, and it was only five now. Maybe he got off work early?<

“Hello...?”

Nothing.

That was weird. Maybe you were just imagining things? 

Turning back, you started the coffee maker and began zoning out.

The sound of shuffling returned, this time accompanied with a heavy musk-like smell. 

Okay, this was really freaking you out.

“Sans, are you trying to pull some sort of weird prank on me? I told you I don’t do well with jump scares, dude.”

Still nothing. 

"…"

That was it. You cautiously exited the kitchen and made your way to were the shuffling sound seemed to have originated. 

Upon seeing nothing at first glance, you made your way to the stairwell, but halted when something moved in your peripheral vision.

Behind the sofa, a small Sans-sized bundle lay on the ground. 

“Sans! You're home! I thought you got off later, why didn’t you answer when I called?"

He made no move to the sound of your voice. His faced the carpet, phalanges pressed firmly to the back of his skull while his chest rose and fell in labored breaths. A soft grown seeped from his throat.

“Sans? 

Hon, you okay?” You coaxed, making your way to his side. The musky smell was stronger now and clearly seemed to be coming from him, making you a little dizzy. 

Still, he didn’t aknowledge your presence.

The closer you got, the more you could see he seemed to be in some sort of pain. Sweat poured from his cranium in rivets. The tips of his phalanges almost looked like they had extended into sharp points and were currently, forcefully, digging into the Parietal bone of his skull making scratch marks as they went. His breath was feverish while he continued to make a sorrowful, pained whine. 

Crouching down beside him you worriedly pressed a hand to his back, trying to get him to look up. He had had panic attacks in the past after particularly bad nightmares. If this was some sort of advanced version of that, you didn’t want to startle him and cause whatever this was to get worse.

One touch was apparently all it took.

Like a rubber band finally snapping under immense pressure, he jolted to attention. Palms slamming into the carpet while his head whipped forward meeting your gaze. Magic saliva leaked from his parted panting maw and a deep guttural growl emitted from his lower throat. The bone that made up the rims of his upper and lower teeth parted revealing sharpened canines and incisors.  


You jumped back slightly, landing on your butt in surprise.

“Sans?”

The growl continued and rose in volume at your voice. 

“Sans... hon, it’s me. Talk to me, are you hurt? Can you speak?”

Gingerly you reached out and cupped his jaw, hoping the gesture would bring him back.

He shivered.

Eye lights staring back at you hazy and unfocused. The growling began to die down, only replaced by a sort of deep chuffing.  


This was getting really concerning…

Not at all his usual behavior during a panic attacks so what-  
…

“Oh…oh god.”

You felt like a complete idiot. How was it not immediately obvious what was happening here? Mentally kicking yourself, you tried to keep your thoughts in order. This had to be his rut right? What else could it be... Everything lined up with your cycle. 

“Fuck,” you whispered. 

You kicked yourself again for not asking either of the brothers the symptoms of the males’ ruts. 

In all honesty you had wanted to forget about it at the time to avoid the embarrassment of entering such a discussion…. You kind of understood what Sans must have been going through when he tried to ovoid the discussion in the past.

You really should have just worked through it though, you should have made a game plan for when this happened. 

Now Papyrus was gone and you were stuck inside with a monster clearly not in control of his thoughts or magic.

“Shit… Sans, Sans are you in there? can you hear me!?”

You were pretty sure your tone went up a few octaves in the last question and you could tell your voice was starting to tremble. 

Hastily, you removed the hand from his face and began to scoot backwards.

He started sniffing the air, eye lights not straying from your form as you moved away.

This was bad. This was really bad.

Rising from the ground, the skeleton monster shuffled forwards on all fours, back arched like a cat stalking it's prey. The animalistic panther-like chuffing continued while he paced the room, tasting the air with his mouth parted. The magic-based drool dribbling onto the carpet as he went. 

A phone…

Where was your phone?  
If you could reach it, you could call Papyrus and ask him what to do. 

This could be resolved, Papyrus would fix this!

Without thinking of the predator watching your every move, you scrambled to your feet, darting to the bag left haphazardly by the kitchen counter.

With hunter like reflexes Sans lunged. 

You didn’t make it far before the wind was knocked out of you from behind as the both of you ceashed into the floor with an audible thud. 

Your vision blurred slightly from the impsct. Head spinning you tried to turn and confront him. Sans skull was inches from you now, saliva spattering across your face with each panting breath he made.

“Sans! 

Sans Get off! You need to listen to me!”

You tried to push him to the side but to no avail.

The struggle seeming to only piss him off as a bony hand with needle sharp claws slammed down on your throat.. 

Gasping, you tried to scream, but the pressure simply increased. The tips of his claws dug into your skin making pinprick nicks in the side of your neck. A small trickle of blood ran down your clavicle. The smell triggering whatever carnal instincts overpowering him to go into overdrive, magic drool now poured from his mouth as he roared into your palling face. 

You weren't getting enough oxygen.

Your vision was beginning to fade and blacken at the edges. 

For a brief moment you feared what would take place if you passed out in this situation. At the least he would end up having his way with your unconscious body and when he snapped back to reality, he would never forgive himself. His fragile mental state surly couldn’t take that kind of trauma. 

At the absolute worse he’d accidentally kill you.  


With the last of your strength you shoved your palm up and out, punching him directly in the eye socket.

That seemed to do the trick.

Screeching, he tumbled off you while clutching his hurt eye.  


Gasping as the air rushed back into your deprived lungs. You scurried backward and up against the wall, still too weak to try to stand.

“Sans…” You coughed 

“S-sans… can you hear me? Are you in there?”

He jolted slightly and froze, turning to meet your questioning stare. His expression becoming slightly more focused by the second. 

“…”

“Y.......y/n?”

...

Y/n wh-... What happened....I 

Sans gaze slowly fell to his clawed phalanges speckled with droplets of your blood, then back to you. Frantically piecing things together, no doubt placing his clouded and confused thoughts into line. 

He began to shake, breath quickening while tears pricked at the rims of his boney sockets.

Oh fuck... Oh fuck oh fuck baby I’m so sorry! are you alright? What the fuck am I saying, no you're not! Shit, y/n I can’t control this, I don’t know what’s happening, I can’t controlthisican’tcontrol-!" 

His claws scraped fervently at his skull as he sank to his knees, breath quickening into a panicked hyperventilating state. Liquid magic streamed down his zygomatic arches, coating the rim if his hoody.

“Sans, *cough*... Sans, I’m… I’m alright. It’s not your fault-"

“Not my fault!? I almost fucking killed you! You could have been dead because of me! My hands are literally covered in your blood! I can’t concentrate, I can’t stop this!”

He was full-on panicking now.

With shaking knees, you made your way to his side and proceeded to rub his shoulders.

“Sans, look at me.”

“I almost killed you, I almost killed you, I almost killed youIamlostkilledyouohgod. I can’t lose you! You and pap are all I have left, oh god I almost killed you!"

“SANS!”

He stopped. The downward spiral of panicked mumbling cut short. Pained eye-lights looked back at you with such helplessness and sorrow, your heart almost burst. This wasn’t his fault. You were both to blame in this situation. He couldn’t control what was happening to his mind and body.

“Sans, I don’t blame you...

I understand…This is the beginning of your rut, yeah?”

“I… I don’t know." he hesitated. "This has never happened to me before. I’ve never been together with someone this long, I-" 

He paused briefly, panic still heavy in his tone.

“You have to leave me. You have to get out now. I don’t know how much longer I can control myself. I don’t want to hurt you, I can’t hurt you again...

Y/n… you don’t know what I’m capable of.”

“…”

“Sans I’m not leaving you like this- ”

“YOU HAVE TO LEAVE! NOW!"

His sudden outburst made you jump. 

Never had you heard Sans shout like that. It scared you, but you couldn’t leave him in this vulnerable state. You were part of the reason he was in this predicament in the first place. This was a new situation for both you and you loved him no matter what happened.

Nothing in life that mattered was supposed to come easy, it had to be worked for, and you’d be dammed if the closest person in your life was ripped from you just because of the fear to see it through. You were filled with determination.

Taking a deep breath you replied.

“No.”

“wha-"

“NO!”  
You shouted back.  
“I’m not going to leave you! This is something we have to work through together. I’m..." 

You paused, still collecting your thoughts.

"...I’m okay with whatever happens next. I love you. I’m not going to give up on you."

Tears continued to stream down his face as he stared at you with a mixture of shock and reverence.

“Baby, I-"

You didn’t waste any more time, rushing to his side and encompassing him in an embrace. 

“I love you, Sans. Whatever happens, never forget that.”

He hiccupped, pressing his face into the crock on your neck. You felt his tears, soaking the fabric of your top while and clutching you close.

…

You weren’t sure how long the two of you sat there on the floor, holding each other. Time seemed to both pass at a snail’s pace and whiz by.

Finally, you slowly pulled away to look at him, running a thumb under one eye socket to wipe away the stray tears. 

“You, uh… you need to have sex to make this go away, right?"

He didn’t answer. But the blue dusting on his cheekbones and the audible gulp he made was confirmation enough. 

“What happens if you don’t?”

“I… I don’t… I don’t actually know. There’s so little info about the original skeleton species before the war. From what I know it was so rare for a skeleton to actually court, anytime there would be a couple they always just went through with the whole process.”

“Hmm,” you thought out loud, “but if you went through with it, the symptoms would go away?”

He gulped again but nodded slowly, looking anywhere but your face.

“Well...” You sighed, a grin forming at the corners of your mouth

“I don’t mind it a little rough”

The breath hitched in his throat.

“Y/n, you have to know what you’re getting into. This… this doesn’t go away after one round, it can last days. I… I won’t be able to stop myself anymore once we start. I could hurt you again.... I-" 

“Shh,” you whispered, “I’ve already agreed to this. I’m not backing out now so just accept it.”

You leaned in and pressed your lips to his teeth. He gasped at first from the contact, then growled, wrapping his arms tighter around you, hungrily returning the kiss. The boney ridges of his teeth acting like lips along with his magic sending tiny sparks through your nerves. 

It didn’t take long before you felt his tongue begin to prod at your mouth seeking entrance. You accepted and the magical appendage waisted no time plunging into your throat, making you yelp slightly in surprise. Trying to keep up with his feverish exploration, you probed your tongue between his teeth, tangling the two together. 

The taste of his magic always amazed you. Both cool and refreshing like snow, but equally warm and inviting with a hint of sweet tomato. He drank so much damn ketchup it seemed to be permanently infused into his magic.  


Moaning along with him, you wrapped your arms around his shoulders.

He reciprocated by trailing his hands down to your waist. Slowly, sliding further forward until you had been seated prominently in his lap, legs resting on either side of his.  


Without any warning he thrust upward, his pubic symphysis rubbing against your closed folds, causing you to squeak in surprise as pleasure spiked up your spine.

Your mouths parted with an audible pop and he slithered his tongue out of your mouth.

You were panting now like you had just run a mile and he didn’t seem any better off at that, thrusting upwords again, eliciting another moan to escape your mouth. 

…

Your movements…

The sound of your voice 

The smell of your arousal.

It all was threatening to cloud his mind again. 

He needed this… 

He needed you. 

He needed the world to know who you belonged to.

He needed to fill you with his magic. 

Needed you to hold his offspring and ensure his future.  


Needed to mate…  


Needed to breed.  


Needed to-

…

You could tell he was slipping back again.

“You still with me big guy?” you whispered

His skull snapped up to meet yours with alarming speed.

“I…” 

He shook his head, "...Yeah. I think so, but… being completely honest here…I’m not sure how much longer I can keep myself together.” 

A whine spilled from his throat while he involuntarily bucked his hips again, but this time you ground down to meet him, causing a full-on warning growl to seep past his teeth.

The realization of what he just did seemed to hit him, flinging a hand over his mouth in shock 

“I’m sorry," he whispered.

You snorted. “It’s kind of hot, actually”

“Wha- really?” 

“yeah, your voice is so low I can feel the vibrations in my chest.”

His sockets lidded, grin suddenly turned into a coy smirk, the corners rising to one side. In that way that made your heart flutter.

“Is that so?” He slurred, blue tongue sliding out from between his teeth to lick along the newly canid-like fangs.  


You just starred and gulped, heat rising to your cheeks as you watched that delicate yet powerful piece of solidified magic leaves a train of saliva in its wake.

"S..sharp T..teeth...hot" You mumbled

"Heh..what?"

"uh.." you coughed, shaking your head now to cover the blush spreading over your cheeks.

Mind if we take this upstairs?.

If we'll be going at this for a while, I’d appreciate being somewhere a little more comfortable than the floor.”

He chuckled

“Anything for my mate,” he purred, making the hair on the back of your neck stand on end. 

Bringing you close to his chest, you felt that familiar tug of gravity as you both sailed through nothingness for a few seconds. 

Then as soon as it began, reality crashed down around you once more. It always reminded you of the feeling you'd get on a rollercoaster that would leave you weightless for the briefest of seconds before gravity caught up.

Desperately, you tried to reorient yourself in the new surroundings.  


He had somehow managed to teleported both of you into laying horizontally on his sheet barren mattress.

Before you had a chance to fully realize the position you were in, he began grinding into you again with new vigor, panting even more than before. Those clawed phalanges rested on either side of your head, the tips dug sharply into the mattress and you could have sworn you heard them rip from the pressure. 

Well... He needed a new mattress anyway.

You had to cover your mouth to stop the noises pouring out. A hard bulge was already present under his shorts and a wet spot was starting to form just under the waistband. 

“I… I rest need this,” he growled, sending vibrations of pleasure again to your core. 

You didn’t have any chance to protest when he swiftly grabbed a handful of your top and ripped the fabric in one swipe. 

Skeleton strength is increased during ruts... good to know. You mentally stuffed that info away for later, glancing at the once decent T-shirt and bra now ripped to shreds and laying in a heap on the side of the mattress. You were silently grateful that was just a work uniform and an old bra which could be easily replaced.

“Fuck, I’m sorry,” he wheezed.

“It’s okay. I wasn’t attached," you whispered breathlessly.

He stopped his rutting long enough to bend down and leave a trail of nips and licks along your neck, moving down the crook of your clavicle, where he began to lap and suck at the sensitive skin.  


One hand trailed up to a breast and began to knead and cup the flesh, using a thumb to flick at the perked nipple.

The other hand traveled down to your lower stomach, massaging the skin there. 

“Gonna fill you up, y/n. Gonna pump you full of my seed."

Stars, I can’t wait to feel my magic pooling right here,” he punctuated by pressing down slightly. 

“You're gonna be swollen with my pups. I’m going to make sure everyone knows who you belong to.” 

You moaned louder, heat flooding to your cheeks while you felt that tell tail slickness coating your thighs. 

He could talk dirty sure, but not like this…  
This was next-level for him.

“You already wet for me? Mmmhhff you’re such a perfect mate. God, I can smell your arousal, It’s so pungent I could get drunk on the scent.”

The hand at your breast continued to flick and pinch, while the other snaked its way down your panty-line, cupping your mons.

Mmmmff, I could smell it you know."

...

The scent of it has been in the house for two fucking days Y/n. ... Your eggs, ready… 

He snaked his tongue across your jaw line. 

...waiting...

...for me.

Your back arched as his thumb brushed the sensitive nub at the apex of your sex.

Maybe it was the heavy smell of his hormonal musk in the air or all that dirty talk, but you couldn’t take it anymore. You needed this. 

You shoved him off you, receiving a questioning grunt in return, as you tried to shimmy out of your pants. He chuckled watching you struggle.

“Ass,” you retorted, finally managing to get the piece of clothing off your body and flung them across the room.  


“You’re adorable,” he hummed, shimmying out of his Hoody and shorts at an impressive speed leaving him in nothing but a T-shirt and his boxers. The ivory of his ribs glistening in the low light.

He wasting no time toping you once more, continuing his rough and fevered dry humping.  


Moving lower, he brought his mouth down to your other exposed breast and sucked down hard.  


Your desperate whines and his panting filled the room. You’d feel self-conscious of the sound if it didn’t feel so good.

“Sans…ahhh…. Sans, please-” 

He grunted and chuffed in response, halting his thrusts. You could tell if you went any further there would be no turning back, this was the last straw.

He gazed into your eyes, searching for any signs of fear– his final silent question if you needed an out.

Bringing a hand up, you cupped his jaw once again, trailing a thumb along the smooth surface. 

“It's okay. I trust you.”

With that-

you lost him. 

His eyes returned to a hazy fog, leaving him purely to his carnal instincts. 

He clawed desperately at the remaining garments covering his upper body like a trapped animal, silently making you hope he wasn’t too attached to those. But ...then again you supposed it was only fair since he ripped yours up.

Crouching down, he started sniffed between your thighs chuffing softly as if inspecting his prize.

“Uh…. what are you—EEHP!”

Suddenly your panties where ripped from your body, usung his teeth to discard the apparently offending article of clothing. He made a more sniffs at your folds now on full display, making you to blush further. 

Any thoughts running through your mind were cut short as he slid his tongue across your lips, saliva sending sparks to your entrance. Moaning again, you shut your eyes as he grunted his approval.  


When he stopped you almost protested, shooting your eyes back open only to land on the reasom why he paused. His boxers lay in a pool around his metatarsals, letting his length spring free.

Sans was never small, exactly. He always had a decent-sized and girth to match, but apparently the rut affected more than just his mental state.

He was huge. 

You weren’t sure you could take that. You took in a deep breath of air. What had you gotten yourself into?

You didn’t have much time to process, yelping as you were flipped onto your stomach.  


He mounted you and lined himself up with your entrance. You gulped. No backing out, you agreed to this.

You inhaled sharply through your nose while the tip of his member found your entrance and began to press inside. Just the head stretched the limits of what you were typically used to. You hissed as he inched slowly further. 

Your nails dug into the mattress. The pain was present, but bearable. 

Then without warning, his hip suddenly snapped forward, making him bottom out inside you. 

You screamed.

Never in your life had you been this full, and for the briefest of seconds, you saw stars companied with the white-hot pain of being filled to your limits. God, he was so deep. Your cervix stretched to accommodate the increased length, but you weren’t sure how much more your body could take.

A deep guttural grown seeped passed is teeth while he lay his chest flat against your back. 

Lapping at your exposed neck where the hair had parted, flowing like a curtain on either side of your face. You were grateful that despite his fogged state of mind he was still giving you a few minutes to adjust.

Your walls spasmed around his shaft, which in turn mase him grunt all the more at how tight you felt. Every small pulse from him sent shivers up your spine and heat to pool in your abdomen. Finally, after a few minutes, he proceeded to ever so slowly slide out and back in, picking up a steady pace. Your arousal seeming to provide enough lubricant to make the movements manageable. 

His arms slid down and wrapped around your middle, giving him more leverage as his pace increased.

Each insertion slipping deep within your core and bumping the small opening at the end of your canal. 

The musk smell from earlier only grew stronger now, filling the room and your senses, clouding your mind in a haze filled lust.

You needed this, too... 

you needed him.

You needed his magic and his seed deep inside you. 

You needed to be one.

Letting out a wonton moan, your tongue slipped lewdly past your teeth.

He reciprocated by adjusting his angle slightly. Lifting your front off the mattress, now completely held in his arms while he started to piston into you. 

The incline was making you see stars and the new angle allowed him to slide even deeper inside you reaching that precious bundle of nerves you had convinced yourself didn't exist.

Sans name fell from your lips in a mantra of reverence and love, joining his needy growls and grunts.

The pressure was building. This felt way too incredible. 

Sleeping with Sans was always amazing, but something about this new monster, driven by this carnal instinct to mate was doing something to your mind and body and you weren’t stopping any of it. 

The room resonated with the wet slapping of his thrusts, again and again, bottoming out inside you while he licked and and nipped at anywhere he could reach. Trailing from your lumbar spine, up to your earlobe.

That familiar coil began to wind itself it the pit of your belly, each blow to the back of your core sending shocks of pleasure throughout your entire body and made your legs shake in desperate need for release. 

One hand moved from its vice-like grip around your midline to massage at your clit.

All it took was a few vigorous rubs and your body began to jolt as wave upon wave of pleasure coursed through you, letting out a screech the likes of which you had never made in bed before. 

Your vision started to spot at the corners from how hard the orgasm hit you.

Again even in his clouded state he somehow knew to slow his speed and expertly ease you through your high. By the time your body jerked for the last time, your poor limbs felt like putty.

Then, his pace quickened back up becoming brutal. Each thrust more and more jagged and untimed then the last. He was barely pulling out now, just rutting and swaying his hips trying to go deeper and deeper within. 

It was too much– you were too sensitive. You whined for him to ease down, but it was no use, he was lost to the whims of his need to breed.

With one last thrust, he drew almost all the way out causing you to shiver-

Then thrust back in as far as he could go with so much force the tip of his member slipped passed the opening to your womb.

For a few seconds, he stayed there frozen. Tears pricked at the corners of your eyes by the sheer intensity of it. But rather than any pain, the pleasure was indiscernible. 

How could this feel good? 

Things weren’t supposed to penetrate there, that wasn’t supposed to-

Suddenly the base of his length started swelling inside you.

“Sans…. Shit, Sans is that a kn-!! AHHH!”

The words died in your throat as he pushed in further, burying the head within your abdomen while he finally came undone, roaring as he simultaneously sank his canines into the exposed flesh at the end of your neck, marking you. 

Spurts upon spurts of hot magic were pumped into your womb.

For a moment it was searing hot, so hot you couldn’t take it. 

Then, as soon as the pain began, it faded, replaced by utter bliss. 

His knot preventing any extra magic from seeping out and the stream of his release didn’t seem to be stopping any time soon. The pressure and swell of your stomach was growing steadily, but the blissful fog clouding your mind at this point was too thick for you to worry about the implications of such.

Finally, after what must have been close to 5 minutes, he stopped spilling inside you, but the knot still kept him from withdrawing. 

Sans slumped down; all energy finally spent.

Carefully, he turned your limp body around and on your side so he could see your face. 

"Gah,” He gulped

“B… baby doll, you okay?”

You couldn’t quite process that he was speaking to you at first. The edges of your mind screaming for rest. The last of your strength finally spent.

“Baby… shit shit shit. Sweetheart… y/n can you hear me?"

You raised your arm weakly into his line of sight, giving him a thumbs up. You heard the wind rush out of his nasal aperture with relief.

“stars, you scared me half to dust.”

“mm…m sorry,” you mumbled

“You alright through? Anything hurt...”

…

Crap” 

His words were cut short, attention suddenly on your neckline.

Cursing again, he brushed a fingers against the mark left on your shoulder. Deep punctures where his canines sunk into your skin.

A thin trickle of blood oozed from the wound.

In your spend state you hadn't registered the lacerations , but now even the feathered ghosting touch brought feeling back to your neck, pain blossoming where he marked you making you hiss at the injury. 

In your peripheral vision a faint green glow emitted from his palms. Slowly the ache began to subside.

“Just sealed the wounds and made it stop bleeding. It’s all I can manage; I can’t heal the scar." 

“It's okay," you croaked. "It’ll be your mark.”

This made his grin grow, filled with a soft love.

“My mark, huh?” He mumbled, pressing his cool forehead with your now sweaty one.

“Mmm…. So....full…" you whined, glancing down at your swollen lower half. It glowed a soft blue from the liquid magic that had been poured inside you.

“Shit…Heh…I really filled you up huh?”

All you could do was whine at that

“Shhh, I know babydoll… it won’t last, your body will absorb it.”

“…”

“Um…

We might be stuck together for a while though....but it should only last an hour or so… 

He grumbled.

“I’m sorry, I wish I could provide you with the proper aftercare right now.”

“Sss okay," you whispered sleepily. "It was amazing. You were amazing. Even if you made me fat.”

He snorted. 

“yeah… but you're absolutely gorgeous like this.” he murmured resting a palm lovingly to the bulge, massaging small circles into the flesh there. He maintained an expression of something resembling pride while he gazed lovingly at what he had done to your body. 

aHe pulled your limp form close to his chest and rested his head atop yours. Listening to the raped pace of your heart.

You huffed trying to hide the blush on your cheeks.

“Get some rest, love,” he whispered in your ear. 

....  
..  
.

you're gonna need it.”

Dear Lord, this was going to be a long week.

**Author's Note:**

> Hey guys. If you've made it this far without running away in disgust I thank you! I'm impressed you stuck with whatever this was until the end honestly. Again this was kind of just an experiment to get my feet wet and see if I could potentially write more? I have a few other fic series in mind that I hope to write eventually.
> 
> Some doodles bassed off of this https://twitter.com/TheSheWolfDen/status/1313379426424430592
> 
> You're probably wondering too "If skeleton monster couples can only have one kid then what about our favorite bone brothers!?"
> 
> To answer that:
> 
> I hc that the original skeleton race was eradicated during the monster human war leaving Gaster as the soul survivor of his kind.
> 
> In an effort to recreate skeleton monsters he formed an artificial one using a portion of his soul and physical matter.
> 
> The first attempt was sans, names so for his unsettling lack of HP.
> 
> Though incredibly intelligent Gaster deemed Sans as a semi failure, trying again creating papyrus several years later. 
> 
> Either he was unable to create a third attempt due to the amount of damage his soul sustained, or he fell into the Core before he had a chance.


End file.
